The present invention relates to methods and apparatus for driving members into the ocean floor. It has particular relevance to driving piles, as in the construction of off-shore platforms for oil and gas wells, and also relates to driving casings and conductors for such wells.
It is often desired to drive a member into the ocean floor to depths of several hundred feet or more. Common examples of such members are pilings that support a platform from which oil and gas wells are drilled and operated. Other examples are casings within which an off-shore well is drilled and conductors that contain conduits through which oil and gas flow upwardly to the surface.
The present state of the art calls for pounding the member into the ocean floor by the repeated blows of a hammer. Each blow may contain more than one million foot pounds of energy, but at deep penetrations drives the member only a fraction of a foot.
Piles for off-shore platforms serve as a good example of the state of the art of driving such members, although certain unique problems are involved. The piles for these platforms are usually driven about 200 to 500 feet into ocean floor, depending on the type of soil, the water depth and the expected loads due to storms and other forces. Some of the more recently proposed deep water platforms are of the guyed tower type in which guys anchored to the ocean floor take horizontal loads and the piles of the structure take vertical loads and the horizontal loads at the mud line. Some such proposals call for flexible piles that permit significant horizontal movement at the top.
The pile is driven in sections, typically 80 feet or more in length. A hammer and its leads, which may weigh 600 tons or more, must be supported above the pile by a crane mounted on a barge. The further into the ocean floor the pile is driven, the greater the force required to drive it and the larger the hammer must be. Some experts believe that a large portion of the hammer energy is absorbed by radial movement and vibration of the pile throughout its length.
Each successive pile section is welded to the one that precedes it. The new section must be held by a barge-mounted crane and suspended above the preceding section to which it is to be attached. A stabbing guide must be attached to the bottom of the new section to facilitate its insertion.
As the new section is positioned, the beveled ends of the sections that facilitate welding are easily damaged. The difficult and time-consuming welding operation, that requires precise positioning of the sections, is hindered by the tendency of the new section to move relative to the preceding section as the barge moves with wind and water currents. The direct effects of wind and water spray on the welding equipment can make welding impossible for long periods of time, even if the positioning problems can be overcome.
Many areas in which platforms are located, including, for example, the North Sea, frequently have severe storms. It is, therefore, necessary to wait for a suitable "weather window" during which to erect the platform and drive the piles. As the water depth and the time required to drive the piles increases, the necessary window becomes larger. The difficulty of finding such a window increases as does the chance of an unexpected storm that could prove disastrous. It is important to drive the piles as rapidly as possible so that the structure can withstand heavy seas, if necessary.
All the above limitations, the hammer size requirements, the necessary weather window and the maximum available size of cranes, barges and other support equipment, collectively known as the spread, place a practical upper limit on the water depth in which off-shore platforms can be located. At present, there are only a few platforms in as much as 1,000 feet of water. The demand for oil from many deep water locations in which it is known to exist cannot be met without new concepts and basic improvements in the method and apparatus by which piles are driven.
The tallest platform structures contemplated today are of the guyed tower type in which the piles are intended to bend rather than resist horizontal loads. In structures of this type, the problems arising from the use of a hammer are compounded since piles having the desired flexibility will absorb a large portion of the hammer energy and it may be impossible to drive the piles to the desired penetration.
Apart from the size limitations of the technology in use today, there are other disadvantages associated with conventional hammer-driven piles that relate to their essential purpose of securing the platform. When the pile is hammered, it unavoidably moves radially as it abruptly surges downwardly with each blow. In so doing, it disturbs the soil around it, and may leave an annular space between the pile and the soil which reduces soil friction. Although the soil may regain part of this initial strength as it settles, some loss is permanent. The result is that the forces and energy required to remove the pile are less than that required to drive it and the holding power of the pile is not accurately predictable, even if the energy used in driving it is known.
A problem experienced with hammer-driven piles is that the numerous variables make it difficult or impossible to accurately monitor the force required to drive the pile at successive penetration levels. For this reason, existing techniques that attempt to predict the static-bearing capacity of a pile based on the history of its dynamic driving resistance are not totally reliable. To compensate for this unrealiability, large safety factors must be included in design specifications. In some situations, a pile is driven at considerable cost to a predetermined depth far greater than that required to secure the platform when soil conditions offer more resistance than expected.
Objectives of the present invention are to provide new methods and apparatus for driving piles and other members more efficiently. A further objective is to utilize apparatus that is of less weight, has lower energy requirements, and is more easily managed, permitting construction at greater water depths. Other objectives are to drive the member in a manner that minimizes the disturbance of the soil surrounding it and renders the holding power of the member more predictable.